The present invention relates to a supercritical refrigerating cycle that utilizes carbon dioxide as a coolant.
An example of a refrigerating cycle utilizing carbon dioxide (CO.sub.2) as a coolant, which is disclosed in Japanese Examined Patent Publication No. H 7-18602, comprises a compressor, a radiator, a counter-flow heat exchanger, a means for expansion, an evaporator, an accumulator and the like.
In this structure, coolant is compressed by the compressor to be a vapor-phase coolant with a high pressure, and then it is cooled at the radiator to reduce its enthalpy. During this process, since the high-pressure vapor-phase coolant is at a temperature equal to or higher than a supercritical temperature (in a supercritical range) of the coolant, it is not condensed and does not become a liquid phase state at the radiator. In this point, the refrigerating cycle is different from prior refrigerating cycles employing Freon. Then, the high pressure coolant with the reduced enthalpy travels through the expansion valve so that its pressure is reduced down to a vapor-liquid mix range, and thus, the liquid-phase component is increased for the first time in the coolant in this stage. Subsequently, the liquid-phase component in the coolant absorbs heat of a medium traveling through the evaporator to be evaporated and then it is taken into the compressor.
In the refrigerating cycle described above, the counter-flow heat exchanger achieves heat exchange between the low temperature vapor-phase coolant taken into the compressor and the high-pressure vapor-phase coolant after passing through the radiator, and since the low pressure vapor-phase coolant is heated and at the same time the high-pressure vapor-phase coolant is cooled at the counter-flow heat exchanger, the efficiency of the refrigerating cycle is improved.
However, as it is a known fact that there is an optimal heat exchanging capacity in a refrigerating cycle employing a counter-flow heat exchanger depending upon the environment in which it is operated or the operating state. It is another known fact that if the environment or the operating state changes, the optimal heat exchanging capacity also changes, and therefore the optimal heat exchanging capacity must be adjusted in order to achieve improved efficiency under varying conditions. However, if the optimal heat exchanging capacity is changed, a problem arises such that the degree of superheat of the coolant in an intake side of the compressor becomes excessive, resulting in a high discharge temperature.
The temperature of the air entering the radiator changes constantly (due to changes in the external air temperature, during idling or high speed operation and the like). Furthermore, the force to drive the compressor is derived from the running engine so that the rotating state of the compressor changes in conformance to the running state. As such, when a refrigerating cycle as described above is employed in an air conditioning system for vehicles, problems arise because the environment or the operating state changes frequently.